JPS6265901A - Thermochemical production of hydrogen from water - Google Patents

Abstract

PURPOSE:To produce hydrogen at an excellent reaction rate, with high heat efficiency and in high yield by improving the process when gaseous hydrogen is thermochemically produced from water by an Mg-S-I cycle method. CONSTITUTION:A gaseous mixture of SO2 and O2 is blown into an aq. slurry at 0-120 deg.C wherein MgO and I2 are suspended in water to form a slurry consisting of an aq. soln. of MgSO4 and MgI2. The slurry is separated by filtration, centrifugation, etc., into an MgSO4 crystal and a concd. soln, of MgI2. The concd. soln. of MgI2 is heated, concentrated and evaporated to dryness. The residue is further heated at 400 deg.C to generate HI and the residue of MgO is returned to the first stage. The MgSO4 obtained by the preceding stage is brought into contact with an aq. soln. and the soln. is heated at 900-1,200 deg.C to generate SO2 and O2 and the residue of MgO is returned to the first stage. Then the HI is brought into contact with an aq. soln. and the soln. is thermochemically decomposed by electrical energy into a gaseous mixture of H2 and I2. The I2 is condensed with use of a separation membrane or by cooling and high-purity gaseous H2 is separated from the I2 and recovered in high yield.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an improvement in a thermochemical hydrogen production method using water as a raw material.

[Prior art]

Conventionally, regarding thermochemical hydrogen production methods using water as raw material,
Various methods have been proposed, among which Mg-5
- Method using I cycle [Special Publication No. 56-7961, S., Mizuja and T., Kumagai
, Bull.

Chem, Soc, Jpn, 55.1939 (1
982)) is a highly practical method, which consists of the following reaction steps.

(1) 2Mg0(c)+5Oz(aq)+I2(C
)−+Mg5O4(aq)+Mgl z(aq)(2)
Mg I z (aq) → Mg○ (c) + 2 HI (g) + nH20 (g)
(3) MgSO4(c) −+ MgO(c)+ SO2(g)+ (1/2)0
2 (g) (4) 2 HI (g) → Hz (g) + I z (g) For the improvement of this conventional method, three reactors are combined in three reaction stages without transferring solid reactants. An operation method that can be used selectively has been proposed (Japanese Patent Application Laid-Open No. 152203/1983, S. Mizut, a and T. Kumaga
i“Hydrogen Energy Progress
V″ProceedingS of t,he
5th World) 1ydrogen En
energy conference, toronto
, o, Canada.

15-20 July, 1984. ρ421).

However, in this method, the concentration of the solution used is low, MgI2 and Mg5O4 are not separated, and H2 and O2
The problem is that the reactor temperature needs to be raised and lowered in order to generate the two in different temperature ranges, and from an industrial implementation point of view, problems such as the thermal efficiency of the cycle and the increase in the size of the equipment have not yet been resolved. .

〔the purpose〕

An object of the present invention is to solve the problems seen in the conventional methods.

〔composition〕

According to the invention, in thermochemically decomposing water.

(a) A mixed gas of sulfur dioxide and oxygen is blown into an aqueous slurry containing magnesium oxide and iodine, and only sulfur dioxide is reacted and absorbed to produce a slurry consisting of an aqueous solution of magnesium sulfate and magnesium iodide, which is then separated. (b) separating the slurry obtained in the above step (a) into solid-liquid to obtain magnesium sulfate and magnesium iodide aqueous solution separately; (c) the above step (b) ) A step of heating and concentrating the magnesium iodide aqueous solution obtained in step (2) to generate hydrogen iodide through hydrolysis, and circulating the magnesium oxide obtained as a residue to the step (a), (d) the step (b)
The magnesium sulfate obtained in step (1) as it is or redissolved to form an aqueous solution is thermally decomposed at high temperature to generate sulfur dioxide and oxygen, and obtain magnesium oxide as a residue, and these products are processed in step (a). The process of circulating.

(e) decomposing the hydrogen iodide obtained in the step (c) into iodine and hydrogen, circulating the obtained iodine to the step (a), and removing hydrogen from the reaction system. Provided is a method for thermochemical hydrogen production from water, characterized by:

Step (a) in the present invention is represented by the following reaction formula.

+ (1/2)02 (g) In this reaction formula (i), the value of n is 8 to 32, preferably 10 to 20. Mg5O4・mH20(c) represents magnesium sulfate anhydrous salt or hydrated salt crystal, m
The value of usually ranges from 0 to 7.

The reaction represented by the reaction formula (i) is carried out at 0 to 120°C.
In this method, a mixed gas of sulfur dioxide and oxygen is added to an aqueous slurry in which magnesium oxide (partially containing magnesium sulfate) and iodine are suspended in water (partially containing hydrogen iodide or magnesium iodide). This is carried out by blowing and reacting and absorbing only sulfur dioxide. In this case, by reducing the amount of water used, a slurry of anhydrous or hydrated magnesium sulfate and a concentrated magnesium iodide solution can be obtained, and at the same time, oxygen can be separated and recovered from the reaction system. In this process, by reducing the amount of water used, it is possible to improve the separation between magnesium sulfate and magnesium iodide, and by adjusting the amount of water to 31 mol or less per 1 mol of iodine, precipitation The yield of magnesium sulfate can be increased to 80% or more, and the mixing of magnesium sulfate into the aqueous magnesium iodide solution can be significantly suppressed. When the amount of water used is reduced to about 20 moles per mole of iodine.

The yield of precipitated magnesium sulfate is close to 100%.

When magnesium sulfate is precipitated from an aqueous solution containing magnesium iodide, it is also possible to precipitate magnesium sulfate hydrate (or Although it is possible in principle to precipitate anhydrous salt), in this case, a so-called homogeneous precipitation method is used, and the magnesium sulfate hydrate is precipitated as extremely fine particles of 1μ0 or less. Therefore, many solid layers are generated in physical solid-liquid separation methods such as filtration. The most significant feature of the present invention is in step (a), in which one reaction (i) is gradually progressed while gradually blowing in SO2 gas, so the generated MgSO4 mH2O particles are several tens of microns.
It grows to a size of about 1.5 m and can be easily separated by physical means.

Next, in step (b), the slurry obtained in step (a) is separated by solid-liquid separation operations such as filtration, centrifugation, and decanting, and crystals of anhydrous magnesium sulfate or hydrated salt and concentrated magnesium iodide are separated. The solution is obtained separately.

In step (c), the magnesium iodide solution obtained in step (b) is heated and concentrated, evaporated to dryness, and then further reduced to about 40%
It is carried out by heating to 0°C. At this time, after evaporation to dryness, hydrogen iodide started to be generated from around 150℃,
During heating up to 400° C., hydrogen iodide is completely released and almost pure magnesium oxide is obtained. This reaction is expressed by the reaction formula (2) above, and the produced magnesium oxide is recycled to step (a).

Step (d) is carried out by heating the anhydrous or hydrated magnesium sulfate salt separated in step (b) to 900 to 1200°C, either as it is or after redissolving it to form a solution. Through this step, the reaction formula (3)
As shown, magnesium sulfate decomposes thermally, generating sulfur dioxide and oxygen (including some sulfur dioxide), and producing magnesium oxide as a residue. These products are recycled to step (a).

In step (e), the hydrogen iodide generated in step (c) is heated as it is or after being made into a sample solution.
Hydrogen and iodine (containing water vapor or water and undissociated hydrogen iodide) are obtained as a mixed gas or separately by decomposition according to reaction (4) using light or electrical energy.

To give an example of decomposition of hydrogen iodide, for example, in the case of thermal decomposition,
In the case of photolysis, it can be carried out by heating hydrogen iodide to 300-1000 °C, by irradiation with ultraviolet light of 250 nm or less, and in the case of electrolysis, it can be carried out at 110 °C or more using a phosphoric acid liquid film as an electrolyte. It can be carried out by electrolyzing at normal temperature to 100° C. after forming an aqueous solution. When hydrogen and iodine are generated as a mixed gas, this mixed gas is separated by a separation membrane or cooled (0 to 120°C) to condense components other than hydrogen.

It can be separated into hydrogen and iodine (including water and hydrogen iodide). The iodine obtained in this step is
Meanwhile, hydrogen is separated and recovered from the reaction system.

〔effect〕

The thermochemical hydrogen production method from water according to the present invention overcomes the drawbacks of the conventional method.
It can be carried out with a high yield of 5% or more, and the reaction rate is high.

(2) In each step of Step 8 (a) to (e), the products can be easily separated.

(3) There is no need to use excess reactants to advance the reaction, and it is sufficient to circulate only the substances directly involved in the reaction. In particular, it will be possible to reduce the number of head offices used, leading to a significant improvement in thermal efficiency.

(4) Since all cycle materials including @ bodies can be transported and circulated and each process can be performed continuously, it is easy to increase the size of the device.

(5) High safety because magnesium iodide, which is a hydrogen generating substance, and magnesium sulfate, which is an oxygen generating substance, are separated.

With these characteristics, the present invention is suitable as a reaction cycle for industrial implementation in which hydrogen and oxygen are continuously extracted through a thermochemical reaction of water.

〔Example〕

Next, the present invention will be explained in more detail based on examples.

Example (A) Step (a) 0.15 mol of magnesium oxide powder prepared by hydrolysis of magnesium iodide and 0.15 mol of magnesium oxide powder prepared by thermal decomposition of magnesium sulfate.
1 is mixed with 0.15 mol of iodine powder, and 2 mol of water is added to this.
．． 5 to 4.5 mol was added to form a slurry. Next, when a mixed gas of sulfur dioxide and oxygen produced by thermal decomposition of magnesium sulfate is blown into this slurry for about 1 hour, the sulfur dioxide is reacted and absorbed according to the reaction (1) above, and becomes magnesium sulfate heptahydrate. A slurry consisting of magnesium iodide solution was formed, and the coloring caused by iodine became lighter.

On the other hand, the outlet gas passed through an alkali absorption bottle and then led to a flowmeter and an oxygen concentration meter set with a zirconia sensor. Regarding the outlet gas, the amount of sulfur dioxide was determined from the amount of alkali consumed, and the amount of oxygen was determined from the integrated value of gas flow rate and oxygen concentration. Comparing these values with the amount of thermally decomposed magnesium sulfate, the absorption rate of sulfur dioxide in reaction (1) is 70-90% or more, while oxygen is hardly absorbed and the two are well separated. It was shown that

(B) Step (b) The slurry obtained in the above reaction was separated using a glass filter, and subjected to X-ray analysis of the solid phase and chemical analysis of the liquid phase (EDTA
Mg titration, free iodine titration, and total iodine titration using sodium hypochlorite solution as the oxidizing agent) revealed that the solid was almost pure crystals of magnesium sulfate heptahydrate, and that the precipitated sulfuric acid It became clear that the yield of magnesium crystals and the number of moles (n) of water used (based on 1 mole of iodine used) had a relationship as shown in the table. That is, in the case of the present invention, the amount of magnesium sulfate mixed into the magnesium iodide precipitated in step (a) can be adjusted by adjusting the amount of water used, and by reducing the amount of water used. , the amount of contamination can be reduced. Therefore.

The present invention is extremely convenient for industrial implementation because magnesium sulfate and magnesium iodide can be separated in a highly concentrated solution that is advantageous in terms of thermal efficiency.

Further, in the step (a), when the reaction was carried out at 90° C., it was confirmed by X-ray analysis that Mg504.H2O was precipitated.

Table (C) Step (c) A part of the magnesium iodide aqueous solution obtained in the above step (b) was collected in a quartz boat E, and it was heated in a nitrogen stream for 40 minutes.
The temperature was raised to 0°C, and the gas generated at this time was condensed and collected in a condenser. It was confirmed by X-ray analysis that most of the residue in the quartz boat when held at 400°C for 20 minutes was magnesium oxide.

Next, the distillate was subjected to chemical analysis (free iodine titration,
When total iodine titration and neutralization titration were performed, it was revealed that hydrogen iodide was recovered with a yield of 90% or more.

(D) Step (d) A part of the magnesium sulfate heptahydrate crystals separated in the above step (b) was collected in a quartz boat, and heated in a nitrogen stream for 100 min.
When the temperature was raised to 0-1200℃ and heated for about 1 hour,
Sulfur dioxide and oxygen were generated according to reaction (3). X-ray analysis confirmed that the residue was magnesium oxide.

(E) Step (E) In the same manner as in (C) above, the magnesium iodide aqueous solution obtained in Step (B) was heated to 400°C, and the generated gas was kept at 800 to 1000°C in a quartz tube. After the decomposition reaction (reaction (4) above) was carried out, it was condensed in a condenser.

Next, this condensate was subjected to chemical analysis (free iodine titration, total iodine titration). On the other hand, after the outlet gas passes through an alkali absorption bottle, it is guided to a flow meter and a thermal conduction hydrogen concentration meter.
The amount of hydrogen generated was calculated from the integral value of gas flow rate and hydrogen concentration. From these results, it was confirmed that 20 to 30% of hydrogen iodide was dissociated.

Claims (2)

[Claims] (1) In thermochemically decomposing water, (a) A mixed gas of sulfur dioxide and oxygen is blown into an aqueous slurry containing magnesium oxide and iodine, and only the sulfur dioxide is reacted and absorbed to form magnesium sulfate. A step of producing a slurry consisting of an aqueous magnesium iodide solution and removing the separated oxygen from the reaction system; (b) solid-liquid separation of the slurry obtained in step (a) to separate magnesium sulfate and an aqueous magnesium iodide solution; (c) heating and concentrating the magnesium iodide aqueous solution obtained in the step (b) to generate hydrogen iodide through hydrolysis, and converting the magnesium oxide obtained as a residue into the step (a); (d) The magnesium sulfate obtained in step (b) is made into an aqueous solution as it is or is redissolved, and then thermally decomposed at high temperature to generate sulfur dioxide and oxygen, and leave magnesium oxide as a residue. and these products were subjected to the above step (
(e) decomposing the hydrogen iodide obtained in the step (c) into iodine and hydrogen, circulating the obtained iodine to the step (a), and removing hydrogen from the reaction system. 1. A thermochemical hydrogen production method from water, comprising the steps of: (2) The method according to claim 1, wherein in step (a), the amount of water in the aqueous slurry is in the range of 8 to 32 moles per mole of iodine.